Medical devices have been used to deliver therapeutic agents locally to body tissue of a patient. For example, intravascular stents comprising a therapeutic agent have been used to locally deliver therapeutic agents to a blood vessel. Often such therapeutic agents have been used to prevent restenosis. Examples of stents comprising a therapeutic agent include stents that comprise a coating containing a therapeutic agent for delivery to a blood vessel. Studies have shown that stents having a coating with a therapeutic agent are effective in treating or preventing restenosis.
Even though medical devices having a coating with a therapeutic agent are effective in preventing or treating restenosis, many coated medical devices, in order to coat the medical device with the therapeutic agent or in order to control the release rate of the therapeutic agent, also include a polymer. The use of such polymers may have certain limitations.
For example, some polymer coatings do not actually adhere to the surface of the medical device, instead the coating encapsulates the surface, which makes the polymer coating susceptible to deformation and damage during loading, deployment and implantation of the medical device. Any damage to the polymer coating may alter the drug release profile and can lead to an undesirable and dangerous increase or decrease in the drug release rate.
For instance, balloon expandable stents must be put in an unexpanded or “crimped” state before being delivered to a body lumen. The crimping process can tear the coating or cause the coating to be completely ripped off of the stent. Once in the crimped state the polymeric coating can cause adjacent stent surfaces, such as struts, to adhere to each other. Moreover, if the coating is applied to the inner surface of the stent, it may stick to the balloon as it contacts the inner surface during expansion. Such interference may prevent a successful deployment of the medical device.
Similar to balloon-expandable stents, polymer coatings on self-expanding stents can also interfere with the deployment mechanism. Self-expanding stents are usually deployed using a pull-back sheath system. When the system is activated to deploy the stent, the sheath is pulled back, exposing the stent and allowing the stent to expand itself. As the sheath is pulled back it slides over the outer surface of the stent. Polymer coatings located on the outer surface of the stent can adhere to the sheath as it is being pulled back and disrupt the deployment of the stent.
Since many polymeric coatings only encapsulate the surface of the medical device, not only are such coatings susceptible to deformation, they must coat the entire surface of the medical device in order to encapsulate it. Coating the entire surface with a therapeutic agent/polymer coating can result an excessive amount of a therapeutic agent being applied to the medical device. In certain instances, it is desirable that only the surfaces of the medical device that contact the body lumen are coated with a therapeutic agent. However, when the therapeutic agent is dispersed in a coating composition and the coating composition is applied to the entire surface of the medical device, the therapeutic agent is also applied to the entire surface of the medical device. Such use of excess therapeutic agents is costly.
An alternative to coating or encapsulating the surface of a medical device is to create holes in the surface of a medical device and place a therapeutic agent in the holes. Placing the therapeutic agent in the holes reduces waste and the dangers associated with coating a medical device with an excessive amount of a therapeutic agent since the therapeutic agent is placed were it is needed, instead of on the entire surface of the medical device. However, creating holes in the medical device can negatively impact the structural integrity of the medical device. For example, if holes are created in stent struts, the structural integrity of the stent struts can be compromised causing the struts to become weak. Weak struts could result in the stent failing to expand properly or, once implanted, collapsing, potentially causing re-occlusion of a body lumen.
Accordingly, there is a need for coatings for medical devices that allow greater control over placement of a therapeutic agent on the surface of the medical device as well as coatings that are not easily deformed or damaged, particularly during loading, deployment or implantation of the medical device. Moreover, there is a need for medical device coatings and a way to apply them that does not affect the structural integrity of the medical device.